The present invention relates to a semiconductor device, and to a process for fabricating a semiconductor device, comprising a step of forming a barrier metal layer by means of chemical vapor deposition (CVD).
In general, a large number of connection holes are necessary in a semiconductor device. The term "connection holes" as used herein refers collectively to contact holes, through holes, and via holes. These connection holes are commonly formed by, for instance, fabricating an insulating layer on a semiconductor body (referred to simply hereinafter as "a base") comprising a semiconductor substrate having thereon a lower conductive layer such as an impurity diffusion layer, etc., and then, after providing an opening to the insulating layer, filling the opening with a metallic interconnection material. With increasing level of integration in the semiconductor devices, however, a finer design rule is required. Hence, the development of a technology for filling an opening having a still higher aspect ratio with a metallic interconnection material is keenly demanded.
In a state-of-the-art technology, an opening is filled by depositing an aluminum based alloy by means of sputtering. The term "an aluminum based alloy" as referred herein encompasses pure aluminum and other aluminum alloys. In the deposition using sputtering, the destruction of the lower conductive layer due to the generation of silicon nodules and aluminum spikes is prevented from occurring by first forming a barrier metal layer utilizing titanium nitride (TiN) and the like, and then forming an aluminum based alloy layer on the resulting barrier metal layer.
In case of utilizing sputtering for the deposition, however, because of the so-called "shadowing effect", it can be seen that the deposition of the sputtered aluminum based alloy or TiN on the bottom portion of the opening or onto the side walls in the vicinity thereof becomes more difficult with increasing aspect ratio of the opening. The so-called "shadowing effect" refers to the particular phenomenon which occurs during sputtering. More specifically, the sputtered particles tend to deposit only on optically bright portions while leaving over the shadow portions corresponding to the side walls or the bottom portions of the openings uncovered by the sputtered material. The structure which is obtained as a consequence of the so-called shadowing effect suffers openings having a barrier metal layer with inferior step coverage or an aluminum based alloy layer deposited at poor step coverage on the bottom portions or the side walls in the vicinity thereof. Such defective portions with defective step coverage impair the barrier effect of the barrier layer or result in the generation of disconnection.
To prevent the aforementioned problems associated with the poor step coverage from occurring, a high temperature aluminum sputtering technique is proposed to increase the step coverage of the aluminum based alloy layer. More specifically, referring to the schematically drawn FIG. 7 (A), an insulating layer 14 comprising SiO.sub.2 is formed on a base 10 comprising a silicon semiconductor substrate, and an opening 16 is perforated in the insulating layer 14 to expose the source/drain regions 12 formed in the base 10. Then, as shown in FIG. 7 (B), a contact layer 20 made of titanium (Ti), a barrier metal layer 22 comprising TiN, and a wettability improving layer 24 made of Ti are formed. The function of each of the contact layer 20, the barrier metal layer 22, and the wettability improving layer 24 is described hereinafter.
An aluminum based alloy layer 30 is formed thereafter on the wettability improving layer 24 employing the high temperature aluminum sputtering process. More specifically, the aluminum based alloy is deposited by sputtering onto the base (a semiconductor substrate and the like) being heated to a high temperature in the range of from about 400.degree. C. to the melting point of the aluminum based alloy. By thus depositing the aluminum based alloy layer and maintaining it in a molten state on the barrier metal 22 formed on the insulating layer 14, the fluidized aluminum based alloy can be flown into the opening. In this manner, a connection hole 34 can be formed by filling the opening 16 with an aluminum based alloy. At the same time, the aluminum based alloy layer 30 on the insulating layer 14 is planarized to form the interconnection layer (FIG. 7 (C)).
The high temperature aluminum sputtering method as described above is certainly effective for favorably filling the opening 16 in the aluminum based alloy layer 30. However, it can be seen from FIGS. 7 (B) and 7 (C) that the step coverage of the barrier metal layer 22 and the like remains unimproved. Thus, the problems described in the foregoing, i.e., the impaired barrier effect of the barrier metal layer and the generation of defective disconnection in the barrier metal layer on the bottom portion or on the side walls in the vicinity of the bottom portion, remain unsolved.
In addition to the measure above, collimator sputtering method has been proposed recently. The newly proposed method comprises depositing the aluminum based alloy with a so-called collimator installed between the target and the base. A collimator is a plate comprising a plurality of penetrating holes. Thus, the collimator adjusts the flight of the sputtered particles from the target to the base in such a manner that the angle of incidence of the particles with respect to the base is controlled within a predetermined range. The step coverage in the bottom portion of the opening can be ameliorated in this manner. Still, this method is not always satisfactory. The low rate of film deposition and the generation of particles ascribed to the sputtered particles which adhere to the collimator are problems yet to be solved.
As another means of overcoming the aforementioned problems, there is also proposed a technique of forming a TiN film as a barrier metal layer by CVD. This method based on CVD has attracted much attention as a promising one because it can provide a conformal TiN layer having a favorable step coverage even in openings having high aspect ratio.
In case of forming an aluminum based alloy layer by high temperature aluminum sputtering on a barrier metal layer deposited by CVD, however, the following problems are found to be overcome.
The TiN crystals deposited by sputtering exhibit crystallographic (111) orientation, whereas those deposited by CVD result in (200) orientation or in random orientation. The aluminum based alloy crystals are generally oriented along the crystallographic (111) direction in such a manner that the close-packed plane may be arranged in parallel with the surface of the base. It can be seen that the TiN crystals having the (111) orientation are in good conformity with the aluminum based alloy crystals oriented along the crystallographic (111) direction. Thus, it can be seen that the (111)-oriented aluminum based alloy layer matches favorably with the underlying TiN barrier metal layer also exhibiting the (111) orientation to provide a flat and smooth aluminum based alloy layer with less surface irregularities.
In case of depositing an aluminum based alloy by high temperature aluminum sputtering on randomly oriented TiN crystals or on those oriented along the crystallographic (200) direction, on the other hand, the orientation of the aluminum based alloy is disturbed due to the lattice parameter mismatch. More specifically, the aluminum based alloy crystals tend to exhibit a weak (111) orientation. Thus, a roughened surface results on the aluminum based alloy layer (FIG. 8).
The presence of an aluminum based alloy layer having a rough surface is a great hindrance in realizing an accurate mask alignment upon forming the interconnection by applying photolithography to the interconnection layer. Furthermore, such a roughened surface of the aluminum based alloy layer brings about halation of light during the exposure of the interconnection layer in patterning the interconnection by photolithography. Even if the interconnection were to be established, the resulting aluminum based alloy layer which constitutes the interconnection suffers severe surface irregularities as to impair the process controllability in the subsequent steps of forming the interlayer insulating layers. Moreover, the resistances of the interconnection against electromigration and stress migration become inferior as to result in poor interconnection reliability.